1. Field of the Invention
The present invention relates to a polyurethane having a thickening effect. It is especially well suited for use in water paints as viscosity agent.
2. Description of the Related Art
In an attempt to reduce the utilisation of solvents in paint compositions, water paints have been developed over the past 15 years, primarily for indoor use. As a rule, these paints contain an acrylate dispersion as binder. With the new water paints it is desirable to achieve, if possible, the same good properties as in paints based on alkyd as binder. This means, among other things, high gloss, water-insensitive film, good levelling and application properties resembling those of alkyd paints. Levelling and application properties are affected to a great extent by the choice of thickener. Traditional thickeners, such as cellulose derivatives and polyacrylate, provide insufficient levelling in these paints. So-called associative thickeners of polyacrylate and poly-urethane type have therefore been developed, which provide excellent levelling. The associative polyacrylate thickeners are supplied as an acid aqueous dispersion. One draw-back of these products is their sensitivity to water and alkali. The associative polyurethane thickeners do not suffer from this drawback and, therefore, are extensively used as thickeners in gloss paints.
U.S. Pat. No. 4,079,028 discloses an associative, linear polyurethane thickener containing two hydrophobic end groups and in the polymer backbone hydrophilic ether groups and hydrophobic isocyanate groups. European Patent Applications 127,950 and 307,775 describe associative polyurethane thickeners obtained by reacting a polyisocyanate with a polyetherpolyol, a modifier having two reactive groups and one end-blocking agent. The latter European patent application also states that the modifier should contain a pendant hydrophobic group having 10 carbon atoms. From the production method appears that these pendant hydrophobic groups are preferably located at the end groups.
From U.S. Pat. No. 4,426,485 it is also previously known to enhance the thickening effect of a polyurethane by providing it with hydrophobic segments containing at least two monovalent hydrophobic groups. All these thickeners have however proved, even at low concentrations in water, to give high viscosities. This applies in particular to the types of polyurethane thickeners disclosed in European Patent Applications 127,950 and 307,775 as well as U.S. Pat. No. 4,426,485.
The aim of reducing the amount of solvent in water paints entails however that high viscosity in water for a polyurethane thickener is not desirable for two reasons.
First, one wishes to be able to supply the polyurethane thickener dissolved in water with a maximum dry matter content and minimum viscosity. If the viscosity of the polyurethane thickener in water is too high, it may become necessary to use a combination of water and organic, water-miscible solvent to reduce the viscosity. Commercial polyurethane thickeners therefore generally contain propylene glycol or butyl carbitol to achieve a dry matter content of 15-40%.
Second, one wishes in paints, and especially gloss paints, to have a predetermined application viscosity at a shear rate of about 10,000 s.sup.-1. The viscosity at low shear rates, traditionally measured as a Stormer viscosity, however becomes too high with the associative polyurethane thickeners having high viscosity in water. Although the viscosity of a gloss paint at low shear rates can be reduced by adding surfactants or solvents of butyl carbitol type, these two approaches however entail considerable drawbacks in respect of the properties of the paint. Therefore, it is important to use thickeners which impart the right viscosity to the paint, both at low and at high shear rates.